Apparatus for fractionating natural gas



Nov. 21, 1944. J. LqHALL APPARATUS FOR FRACTIONATING NATURAL GASES 2 Sheets-Sheet 1 Filed June 3, 194 0 101;", L; Hall.

Nov. 21-, 1944. J. L. HALL 2,363,317

APPARATUS FOR FRACTIONATING NATURAL GASES Filed June 3, 1940 2 Sheets-Sheet2 era-Ear:

JOHM 4 H444 Patented Nov. 21, 1944 APPARATUS FOR FRACTIONATING I NATURAL GAS John L. Hall, Charleston, W. Va., assignor of onehali' to Danciger Oil and Refineries, Inc., Fort Worth, Tex., a corporation of Texas Original application December 28, 1939, Serial No.

311,415, now Patent No. 2,295,211, dated September 8. 1942. Divided and this application June 3, 1940, Serial No. 338,601

9 Claims.

This invention relates to an improved apparatus for fractionating natural gas.

This'application is a division of prior application Serial No. 311,415, filed December 28, 1939 now Patent 2,295,211, granted September 8, 1942.

In the past many methods have been proposed for effecting the separation of hydrocarbon liquids from natural and artificial hydrocarbon gas mixtures. Such a separation does not present any great difllculty at relatively low pressures, i. e. up to about to 700 to 800 lbs. more elevated pressures fractionation becomes very difllcult. In order to obviate the diiilculties inherent in such high pressure fractionation, it has been the practice to initially reduce the pressure and then to proceed with fractionation at such reduced pressure range.

Such a method of procedure is a distinct disadvantage in many types of operation as for example, in cyclic processes in which natural or casinghead gas is withdrawn from 'a well for extractio of certain constituents and the residue recompressing the gas for repressuring the well.

It has been discovered that eminently satisfactory fractionation of these hydrocarbon mix- -tures may be effected under any pressure up to several thousand pounds per square inch and without any reduction in pressure of the gas other than that which is an incident of the static resistance of the treating system.

The striking advantagesvof. such a method of operation will at once be apparent. With such a high pressure fractionation it becomes possible, in most instances, to directly return the residue gas back to the producing horizon with but little if any compression, thus effecting large savings in power cost. Such a system operating at substantially well pressure permits the use of compact apparatus of small volumetric; capacity as compared to earlier systems.

- In order to enable a more ready comprehension of the principles of the invention a typical operation will be described in conjunction with the illustrative apparatus shown in the accompanying drawing. In the drawing Figure l is an elevation view of fractionating tower and asso ciated units and Figure 2 is an enlarged detail of the reboiler unit of Figure 1.

However, at.

- Figure 3 is an enlarged detail, partly in section of the coolers and associated parts.

As shown in the drawing, the essential elements of the apparatus unit include a fractionating tower A, a reboiler unit B, connected with the base of the tower and a preheater unit C. The fractionating tower A may be of any suitable size and in the illustrative embodiment comprises a tower about 53 ft. high and about 6 in. in diameter. This tower preferably is packed with iron Raschig rings and is adapted to be maintained under pressure and, in conjunction with the preheater and reboiler, serves to fractionatevthe incoming gases under substantially the pressure obtaining in the well. The unit may be connected to a well through the line I.

As shown in the drawings this line enters the heat exchanger 2 and is connected at its discharge end to the gas line 3 which communicates with the tower A at a low point and below baflle l. Thegases passing through the line iare adapted to be preheated in the heat exchanger 2 by hot fluids circulated to and from the reboiler unit.

Positioned adjacent the base of the tower is a reboiler system including the inclined reboiler tank 5 and the upper communicating tank 6. Tank 5 preferably is of the shell and tube type and is provided internally with a bank of tubes 5' to heat the condensate passing therethrough. A heating medium, such as steam, may be circulated through the shell by means 01 the connections l and 8. The unit 5 is connected at its lower end through the line 9 to the bottom or kettle section of the tower A and at its upper end through line ill to the tank 6. The reboiler tank 5 is also connected to the tank 6 through the branch line I I. As will be seen, more fully hereinafter, the reboiler units, including tanks 5 and the base of the tower.

The reboiler system is interconnected, with the heat exchanger 2. For this purpose line I3 is connected with the upper section of line H at one end and to the shell of heat exchanger 2 at the other. Preferably the leveling box I4 is interposed in the circuit. Hot fluids introduced from the system through line 8|.

pressure control valve l1 into the upper cooling section of the tower; The'remainder of the hot liquid products in the heat exchanger are passed through pump i8 and line It into an intermediate section of the tower A to serve as a scrubbing liquid. 'It is particularly to be observed that theinlet for the reflux liquid is positioned below the inlet of gas line It.

The upper section of the tower A comprises the serially arranged coolers and 2|. These are preferably 'of the internal tube type. Connected to the cooler are the inlet and outlet lines 28 and 23' respectively, through which the cooling medium, such as water; is introduced and withdrawn. interposed between coolers 20 and 2| is an intermediate section 21- The tower sections 2ll,-2| and 22 are shown as being detach'ably secured; if desired these may be welded. In operation, gases which pass upwardly through the tubes of the condenser 20, are

cooled by indirect contact with the cooling medium and the gaseous productsare then discharged into the line 25.- Condensate collecting in the intermediate section 22 may be fed back through line 24, controlled by valve 24 to the upper section of tower A as shown.

The gases and vapors which are preliminarily cooled in the cooler 20 pass outwardly of this cooler through line 25' and are introduced into the top of the cooler 2|. In passing downwardly through the interior tubes oi this cooler the gases Q are further cooled by indirect heat exchange with an expanded and cooled fraction from the preheater introduced through the expansion valve i'i. Expansion of the liquid products passing through line l6 commensurately reduces the temperature to give a refrigeration cooling.

The condensate and vapors formed in the cooler 2| as a result of this expension and cooling is withdrawn through line at controlled by valve and passed through a suitable stabilizer (not shown) to recover the valuable constitu ents,. as for example a natural gasoline fraction.

The cooled gases, consisting largely of methane and ethane, are withdrawn from the lower end of the tubes in 'cooler 2| and discharged Pressure may be maintained on this line by valvesil' and 34".

In one method of: operation, high pressure gases from line 3| may be discharged directly into a residue well through the line SI and branch line 84, controlled by valve 34'. When it is desired to increase the pressure on these gases, prior to delivery to a well, line 3| may be connected to the intake side of compressor It by opening valve 3| and the gas at any desired higer pressure may be forced into a well through line 35. During the operation of the system, the excess of the liquid fraction accumulating in the reboiler system may intermittently or continuously be removed through line 38 controlled by valve 38' and subjected to any desired type of treatment. It may for example be passedto a stabilization unit or to a unit for the production of polymer gasoline.

It will be appreciated that the described apparatus enables the operation of a novel and highly effective process. This process may be employed for treating natural gas for the recovery of heavier valuable constituents, e. g.

propane or butane and the like, and repressuring a producing horizon; similarly the system may be employed for processing a wet gas to recover a casinghead gasoline fraction as well as other valuable heavy constituents of the raw gas and the residue reintroduced into the producing a horizon. It will be understood that when the process is utilized for treating casinghead gases a separator isinterposed between the well and tower to remove the heavy oil prior to fractionation of the gases. 1

The operation, in sharp contradistinction to prior methods, is carried out at well pressure (or if desired at higher than well pressure). In carrying out the process, the tower A is first charged with a mixture of butane, propane and pentane introduced through line 40. Raw gas from a primary source, such as a gas well, and, a in a typical case, at a pressure of substantially 2700'lbs.,' and atemper'ature of approximately 80 .1 is conveyed through line i and thence 20 through the internal tubes in the heat exchanger 2. During passage through the heat exchanger the gas is preheated and is then discharged through line 3 at a temperature of substantially 140 F. into the lower portion of the tower be low the baflle 4 In the tower the upwardly flowing gases are contacted with a countercurrent flow of scrubbing liquid which is continuously introduced into the tower through line iii. Due to such direct contact, the liquid refluxing medium largely condenses the heavier con- 'stituents of the gas and returns these to the base of the tower.

The heavier condensed hydrocarbons accumulating in the base of the tower flow by gravity into the reboiler tank 5 through line 9. In

the reboilerthe condensate is heated by indirect contact with the steam and the temperature oi the condensate is raised to the order of '230 F. Due to the increase of temperature and 40 to the expansion of occluded gases, thermal circulation is initiated and the heated liquid flows upwardly through line iii into the denuder tank 6 and back to the reboiler through line H. During this cycle apredetermined portion of the circulating stream is withdrawn through line i3 and passed to the preheater 2 in the manner described. The quantity which is introduced into the preheater 2 is equivalent to that withdrawn from lines 30 and 3t.

' It will be appreciated that this method of split or double fractionation is as unique as it is effective. It will be observed that the liquid products from the base of the tower pass through the restricted areas of the tubes of reboiler '5 and after receiving a considerable increment of heat are passed into the relatively large area in denuder tank 8. The velocity of the stream is therefore diminished in tank 6 and the lighter lighterends are reintroduced into the tower at H a point spaced from the primary fractional condensation zone. The reboiler unit also serves to add a considerable amount of heat to the concoming raw gas.

In the operation thus far described, it will be seen that the lower section of the system com- 15 prises .two concurrently operating fractionating densate for the purpose of preheating the in-' systems. In the first, in the lower part of the tower itself the raw gas enters at well pressure and at a definite preheat. This stream of raw thermally expanded gas is refluxed for a predetermined portion of its travel during which passes'upwardly and are cooled and denslfied by direct contact with a cooling liquid introduced of about 60 F. In further passage through the cooler the temperature of the gas is again reduced with resulting knock-back of heavier entrained constituents.

In the final stage of cooling. the gases passing downwardly through line and the tubes in cooler 2| are subjected to refrigeration cooling. The cooled gas consisting essentially of methane is thus reduced to a temperature of approximately 5 F. and is discharged through line 3| for utilization as previously described. The condensate and vapms formed in cooler 2| as a result of the expansion and cooling of liquid from preheater 2, and at a temperature of substantially 5 F., are withdrawn through line and treated as desired, for example by passage through a preheater and to a suitable stabilization unit whereby desirable products such as and consequently expanded and then refluxed in a primary condensation zone with a continuously circulating stream of reflux liquid from which liquid the lighter constituents, especially methane, are largely removed, before recycling. In these circumstances the recirculating stream of reflux liquid not only serves as a condensing medium but also, in a sense, as a methane denuding or scavenging medium.

Another salient advantage of the present method resides in the physical character of the eiiluent gas. Since a high partial pressure of methane obtains in the upper portion of the tower and since the system is carried at a very elevated absolute pressure the gas discharged from the system is of high density. When this gas is fed directly to a well, or through a compressor and thence to a well, this high density greatly facilitates the gas flow down into the well.

The improved apparatus, as noted, insures the fractionation of hydrocarbon gas mixture at high pressures. While a typical operation at 2700 lbs. per sq. in. has been described, it will be un- On en- 'tering, the raw gas is immediately preheated derstood that the process is available for employment at a wide range of pressures, i. e. from the order of 700 lbs. per sq. in. up to several thousand pounds;

It will be appreciated that within the broad concept of the invention many permissive variations in mechanical arrangements of the apparatus and in operative technique are possible. All such modifications which invoke the broader principles of the invention are comprehended in the appended claims.

Iclaim:

1. An apparatus for fractionating high pressure natural gas which'comprises an elongated fractionating tower, a line leading from a gas well to the tower, a heat exchanger in said line,

a line to withdraw condensate from the basev of the tower, means to heat such condensate and to force the heated condensate through the heat exchanger directly to the tower to contact preheated gases passing therethrough.

2. An apparatus-for fractionatinghigh pressure gas which comprises a fractionating tower, means to pass a stream of gas to the tower, a preheater associated with said means to preheat the gas passing to the tower, a reboiler unit con nected with the base of the tower, means separately to withdraw a condensed liquid fraction and a gas fraction from the reboiler unit, means to admit the gas fraction to the tower at an intermediate section thereof and means to admit the condensate fraction to the tower at a point below the point of admission of the gas fraction.

3. An apparatus for fractionating high pressure hydrocarbon gases which comprises an elongated fractlonating tower, a gas line for passing high pressure gases to the lower section of the tower, a heat exchanger interposed in the line to preheat incoming gas, a reboiler unit connected with the base of the tower and adapted to fractionate condensate fed thereto from the tower into a condensed liquidfraction and a gas fraction, meansto pass the liquid fraction vfrom the reboiler unit through the heat exchanger and thence to an intermediate section of the tower to serve as a refluxing medium and means to pass the said gas fraction back into the tower.

4. An apparatus for fractionating high pressure hydrocarbon gases which comprises an elongated fractionating tower, at gas line for passing high pressure gases to the lower section of the tower, a heat exchanger interposed in the line to preheat incoming gas, a reboiler unit connected with the base of the tower and adapted to fractionate condensate fed thereto from the tower into a condensed liquid fraction and a gas fraction, means to pass the liquid fraction from the reboiler unit through the heat exchanger and thence to an intermediate section of the tower to serve as a refluxing medium and means to pass the said gas fraction back into the tower at a point above the point of admission of the said refluxing medium.

5. In an apparatus of the type described in which high pressure hydrocarbon gases are fractionated in a fractionating tower, a reboiler unit for fractionating condensate comprising, a reboiler connected to the base of the tower, an enlarged chamber in fluid communication with the reboiler, a gas line connected at one end to the chamber and at the other end to the tower; means to pass a heating medium in indirect heat exchange relationship with the condensate in the reboiler to eflect cyclic thermal flow 0! condensate from the reboiler to the chamber and back .to the reboiler while continuously withdrawing evolved li'ghter constituents of the condensate through said gas line.

6. An apparatus for fractionating high pressure hydrocarbon gases comprising a i'rictionating tower in which gases are maintained at substantially well pressure and 'are subjected to condensation, means to denude the condensate of lighter normally gaseous constituents; means to split the denuded condensate into two streams, means to feed one of said streams to a lower portion of the tower to serve as a scrubbing liquid for incoming raw gases and means including a pressure reduction valve to feed the other stream to a cooler in an upper portion of the tower for indirect heat exchange with rectified gas and means to separately withdraw from the tower the rectifld gas and the said other stream.

7. An apparatusfor fractionating high pressure hydrocarbon gases comprising a fractionating tower in which gases are maintained at substantially well pressure. and are subjected to condensation, means to denude the condensate of lighter normally gaseous constituents; means to split the denuded condensate into two streams, means to feed one of said streams to a lower portion of the tower to serve as a scrubbing liquid for incoming raw gases and means including a pressure reduction valve to feed the other stream to a. cooler in an upper portion of the tower for indirect heat exchange with rectified gas and means to separately withdraw from the tower the rectified gas and the said other stream and to utilize said withdrawn gas for repressur ing a producing horizon.

liquid line connected at one end to the heat ex- 8. In an apparatus for tractionating high pressure hydrocarbon gases a fractionating end or the tower, a gas line connected at one end to the reboiler unit and at the other end to w an intermediate section or the tower, a condensate, line connected at one end to the reboiler unit and at the other end to heat exchan en'a changer and at the other end to an intermediate section of the tower below the said gas line and adapted to feed condensate to the tower for direct scrubbing contact with the preheated raw gas; an expansion chamber heat exchanger positioned in the upper section of the tower, a line connecting the heat exchanger with the expan= 'sion chamber and including a pressure reducing means whereby condensate from the heat exchanger is flashed in said expansion chamber exchanger to cool gases by indirect heat exchange and separate lines to withdraw cooled rectified gas and the expanded condensate respectively.

9. An apparatus for fractionating high pressure hydrocarbon gases comprising a fractionating tower, a gas inlet line connected with the lower portion of the tower, means to heat the gas in transit to the tower, means to condense constituents of the gas in the tower, means to charge a portion of the condensate as a scrubbing liquid into the lower portion or the tower and means to charge another portion of the condensate as a refrigeration cooling medium into the upper section of the tower.

J0 L. HAIL. 

